The present invention relates to latex paints, to polymeric emulsions or latexes useful as binders for latex paints, and to processes for making such emulsions.
Latex paints are mixtures of many ingredients. Typical ingredients include coalescing aids, thickening aids, dispersing aids, defoamers, biocides, pigments, and binders. The large number of ingredients makes it difficult to formulate latex paints. In addition, optional ingredients are also employed in latex paints to enhance various paint properties. For example, rheology modifiers are often employed to enhance the flow and leveling and film build characteristics of a paint.
Flow and leveling is an indication of a paint's ability to form a smooth surface devoid of brush marks, roller marks, or other film thickness irregularities upon application of the paint to a surface. Film build relates to the thickness of a paint film that can be applied in one coat. It is an indication of a paint's hiding power, that is, an indication of how well one coat of the paint conceals a surface.
The viscosity of a pigmented paint usually decreases with increased shear rate, an effect called shear thinning. The relationship is non-linear and is difficult to predict with precision because it is affected by the many different ingredients and processing techniques employed in formulating paints. During application of a paint, for example by brushing, rolling, or spraying, flow is vigorous and shear rates are correspondingly high, on the order of 1000 to 10000 sec.sup.-1 or more. As a result of shear thinning, the viscosity of the paint is low, on the order of 0.1 to 10 poise. Once the paint has been applied, continued flow within the film from leveling, sag, or slump is slow and shear rates are correspondingly low, on the order of 0.001 to 1 sec.sup.1. At such low shear rates, the viscosity of the paint can be as high as 100 to 1000 poises or more.
The paint properties of film build and flow and leveling are not mutually opposed, but it is difficult to achieve both to a desirable degree in a single paint formulation. For acceptable film build, shear thinning must be limited, so that the paint has sufficiently high viscosity, under the high shear rate conditions that prevail in the wet paint film at the point of application, to form a desirably thick film on the substrate. For example, a paint must have a high enough viscosity under brushing conditions that a paint film of the desired thickness will adhere to the surface as the brush moves past. However, for acceptable flow and leveling, it is necessary to avoid excessively high viscosity in the wet paint film under the low shear rate conditions that prevail immediately after the paint is applied. Otherwise, brush marks and other irregularities will remain to mar the appearance of the dried paint film. After a paint has been applied to a surface, the wet film must have a low enough viscosity that the forces attributable to surface tension can cause the paint to flow from thicker to thinner regions of the film to achieve the desired leveling, or uniform film thickness. At the same time, the viscosity must not be so low that the film will sag, slump, or drain excessively on vertical or slanted surfaces under the force of gravity.
Flow and leveling is a property of the paint and is determined by complex interactions among the various ingredients. The low shear viscosity of the polymer emulsion used in the paint is a contributing factor in flow and leveling and also affects other characteristics of the paint, such as its behavior during formulation, flow through pipes, brush loading, and the like. In general, it is desirable to keep the viscosity of the emulsion itself fairly low to allow for flexibility in the formulation of paints having the desired flow characteristics. For example, if the emulsion viscosity is too high, it may be harder to formulate a paint having acceptable flow and leveling.
Emulsion polymerization is a widely used process for making polymer emulsions or latexes. The polymer emulsion is made, for example, by charging the monomeric ingredients, water, and a surfactant into a reaction vessel, emulsifying the monomers, purging the reaction vessel with an inert gas to remove oxygen, and heating the reaction vessel to the reaction temperature. An initiator is then added to the reaction vessel, and the reaction is continued for about 2 to about 4 hours. After the reaction is completed, the reaction vessel is cooled. This synthesis yields an aqueous polymeric composition comprising polymer particles suspended or dispersed in water.
Chain transfer agents can be added to the reaction vessel to lower the molecular weight of the resulting polymer. For latexes that are used as conventional surface coatings, it is usually desirable to maximize molecular weight, and so chain transfer agents are not used. However, when it is desirable to produce a polymer emulsion having lower viscosity, or where shorter chain length is desired for other reasons, minor proportions of chain transfer agents can be used.